When there are large amplitude variations in an input signal, difficulties may show up when the amplitude of the input signal is to be tuned to the dynamic range of the input element of the receiver. This becomes a crucial problem, for example, in the case where the input element is an analog-to-digital converter. Situations may occur in which the input signal saturates the converter and other situations in which the amplitude of the input signal is too low, which leads to poorly obtained digitized signal. It is thus necessary to check the gain of the input signal by means of an automatic gain control circuit.
For slaving the amplitude of the input signal to the dynamic range of the input element, one currently uses an automatic gain control circuit. To cope with both high and low gains, there are circuits comprising a stage that checks the high gains and a stage that checks the low gains. The document U.S. Pat. No. 4,870,370 relates to such a situation in which the activation of the stages depends on the amplitude of the input signal. Therefore, the power of the received signal is measured after which this power is compared with a window of permitted power values to utilize one or the other stage depending on whether the measured power lies inside or outside the window.
When there are considerable and fast amplitude variations, which is generally the case with signals subjected to a digital modulation, one seeks to avoid that the automatic gain control circuit causes too much time to elapse before an appropriate control is obtained. In effect, during this period of time an incorrect input signal is often obtained.